Known hydrophone arrangements used in remote sensing applications, such as in sea bed arrays and underwater arrays, for example, typically make use of piezoelectric devices that require local active instrumentation. This renders these prior art arrangements more susceptible to detection by unauthorised parties. They also have data and power cabling and pre-amp requirements that make such prior art hydrophone arrays bulky, difficult to deploy and maintenance intensive.
In an attempt to address these issues hydrophones based on fibre optic technology have been developed. However it has been appreciated by the present inventor that the prior art fibre optic-based hydrophones typically offer reduced quality of performance when deployed at depth due to a lack of depth pressure compensation. For example, the acoustic signal that is to be detected by the fibre laser sensor is typically a small pressure wave that will be referred to herein as “dynamic pressure”. By way of non-limiting example, a typical dynamic pressure wave has an amplitude of up to around 100 Pa and is in a frequency range of between approximately 10 Hz and 20 kHz. At depth the device is additionally subject to a hydrostatic pressure that will be referred to herein as “static pressure”. In the ocean this static pressure typically increases by approximately 100,000 Pa for every 10 m of depth. Hence, at depth, the static pressure is typically many orders of magnitude larger than the dynamic pressure. This can cause a hydrophone or sensor system to saturate and become deaf to the significantly smaller dynamic pressure and consequently the desired sound signal is not detected. It also necessitates the usage of excessive optical bandwidth to transmit the resultant signal along an optical fibre cable.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.